Aluminum is used in integrated circuit (IC) processes for isolation purposes and for forming p-wells. The feature that makes aluminum attractive for use in p-well formation or isolation is that it is a very fast diffuser in silicon. The aluminum is typically deposited by ion implantation in which the aluminum ions are accelerated at high voltage and caused to impinge the silicon surface where they penetrate a short distance into the crystal lattice. This is followed by a high temperature diffusion in which the aluminum atoms diffuse further into the silicon body.
In the case of aluminum, the implanted atoms can pass through a silicon crystal with relative ease as an interstitial species. This is known as channeling which produces a relatively great penetration that can be employed to advantage and which, when uncontrolled, can cause a great deal of trouble. Apparently, the silicon crystal lattice is relatively transparent to the energetic aluminum atoms. Channeling can be particularly detrimental when aluminum is being used for isolation or p-well formation. It is desirable to enhance the effective diffusivity of aluminum anisotropically, reduce its channeling and increase the extent to which a population of aluminum becomes electrically active. We have discovered that both theory and experiment yield that a single group III species will exhibit retarded diffusion in the presence of very high concentrations of germanium, when that species is interstitial, but it will display enhanced diffusion when substitutional. Combinations of group III dopants are known to interact with each other. (See U.S. Pat. No. 4,746,964.) That interaction can be used to exercise considerable control over the diffusion behavior of the p-type dopants when germanium is present in very high concentrations. This is disclosed in our patent application Ser. No. 710,646, now U.S. Pat. No. 5,137,838, which was filed June 5, 1991, and titled METHOD OF FABRICATING P-BURIED LAYERS FOR PNP DEVICES. The teachings in the above noted patent and patent application are incorporated herein by reference.